Conventional injection molding processes have been modified to create composite building products that combine polymers with filler materials. For instance, composite shingles have been fabricated in a closed molding process utilizing various combinations of polymers, such as rubber and thermoplastics (e.g., polyolefins, polyvinyl chloride, etc.) and fillers (e.g., glass, stone, limestone, talc, mica, cellulosic materials such as wood flour, rice hulls, etc.), along with colorants, and optionally, suitable UV inhibitors, lubricants and other additives that aid in the molding process and provide favorable physical properties to the finished composite shingles. Two popular types of composite shingles formed in a closed molding process including composite shake shingles and composite slate shingles. Different mold tools are created for each type of composite shingle, each with its own surface texturing or contouring to be imparted to the molded article.
A molding apparatus for creating composite building products generally includes opposed mold tools that combine to form one or more closed molding cavities. While the mold tools are in contact with one another to form the cavities, one of the tools shapes a top surface of the composite building product, and the other tool shapes the bottom surface of the product. Additionally, one or both of the tools in cooperation shapes the side or outer edge of the composite building product. Typically, to deliver the material to be molded to the closed molding cavities, a heated, flowable blended composition of a polymer and a filler is moved under pressure from a port in one or both of the opposed molding tools to a distribution channel formed by the mating mold tools. The distribution channel is formed into the surfaces of the mold tools that face one another, in a similar fashion to the closed molding cavities, and extends from the port to the article molding regions of each mold tool that form the closed molding cavities. More specifically, the distribution channel extends to a perimeter edge of each article molding region where the flowable composition is delivered into the closed molding cavities. The composition is then cured in each cavity under pressure for a sufficient period of time to form the molded article as a composite building product. Subsequently, the mold tools are separated and the molded articles removed.
The use of a distribution channel on the mold tool surfaces creates a molded gating waste piece that must be discarded. Because of the distribution channel configuration, the gating piece extends from the side or outer edge of the molded article. Although removal of the gating piece from the molded article is generally not difficult, a vestige is left behind as evidence that the gating piece was once attached to the article. This vestige is undesirable on either the side edge or the top surface of the molded article, as these surfaces may remain exposed after the composite building product is installed on a structural member (e.g., a composite shingle on a roof). It would be desirable to create a composite building product free of a vestige or other mark that is a by-product of the molding process on the side edge or the top surface of the molded product.